1. Field of the Invention
The present invention relates to an optical disc reproducing apparatus for reproducing information recorded on an optical disc and, more particularly, to an optical disc reproducing apparatus having a function of performing equalizer adjustment therein.
2. Description of the Related Art
As has hitherto been known, in an optical disc reproducing apparatus for reproducing information recorded on an optical disc, such as a CD (compact disk) or a DVD (digital versatile disk), equalizer adjustment is performed so as to compensate deterioration in the waveform of an RF signal, which is a reproducing signal that is electrically read and represents the information recorded on the optical disc. The RF signal contains various frequency components. The higher the frequency thereof, the lower the level thereof. In a case where the levels of the components remain low, jitter of the RF signal increases and gets worse. Also, the frequency components other than necessary frequency components lead to the deterioration of the jitter. Thus, the equalizer adjustment, that is, the adjustment of the frequency characteristics of the RF signal is performed by boosting the low-level frequency components and cutting off the components, whose frequencies are higher than a certain frequency, so as to reduce (or improve) the jitter.
The equalizer adjustment is performed by combining both the adjustment of a cutoff frequency and that of a boost amount. The adjustment of a cutoff frequency is to determine a frequency point at which attenuation of the level of frequency components of an RF signal, which have frequencies that is higher or lower than this frequency point or that is within a certain range including this frequency point, by, for example, 3 dBs from a reference level. The adjustment of a boost amount is to determine an amount by which a gain is increased by boosting an RF signal at frequencies around a certain frequency.
FIG. 8 is a flowchart illustrating the equalizer adjustment in a conventional optical disc reproducing apparatus. The equalizer adjustment is described hereinbelow with reference to this flowchart.
First, at the equalizer adjustment in a case where an optical disc loaded in the optical disc reproducing apparatus is a DVD or a CD, the apparatus commences a still operation of causing a pause state (that is, a state caused when the optical pickup jumps a track and stops after the optical disc makes one revolution) in which an optical pickup is halted at a specific track (or address) on the optical disc in step N1. When it is judged in step N2 that the apparatus is ready for measuring jitter, that is, jitter can be measured, the setting of a jitter measuring circuit provided in an LSI (Large-Scale Integration) constituting a digital signal processing portion for performing signal processing in the apparatus is performed in step N3. This setting of the jitter measuring circuit is that of a circuit enabled to measure jitter in the LSI (that is, the setting of a circuit for performing equalizer adjustment on an RF signal outputted from the optical pickup, which is not converted into a digital signal yet, according to the optical disc).
Jitter is a value (%) that is generally used to evaluate a signal of optical discs. As the jitter increases, the quality of the signal lowers.
Subsequently to step N3, the apparatus proceeds to step N4, whereupon a first search for an optimum cutoff frequency Fc is performed. FIG. 9A illustrates this first search for an optimum cutoff frequency Fc.
As shown in FIG. 9A, it is decided in step N31 which of a DVD and a CD the loaded optical disc is. If the optical disc is a DVD, initialization corresponding to a DVD is performed in step N32. On the other hand, if the optical disc is a CD, initialization corresponding to a CD is performed in step N33.
That is, the apparatus sets a range in which the cutoff frequency Fc is searched for. A start point in this range is set to be an initial value. Then, the cutoff frequency Fc is set at the set initial value in step N34.
Thereafter, jitter is measured in step N35. Then, it is decided in step N36 whether the measured jitter is less than a predetermined optimal jitter. If so, the measured jitter is stored as the optimal jitter in step N37. Upon completion of processing in this step N37, or if the measured jitter is or more than the optimal jitter, a point for searching for the next cutoff frequency Fc is determined in step N38. Similarly, jitter is measured in step N35. If the measured jitter is less than the optimal jitter in step N36, the measured jitter is stored as the optimal jitter in step N37. Such a process is repeated until measurement of jitter corresponding to the last cutoff frequency Fc is performed. Thus, plural jitter values are stored. If processing corresponding to the last cutoff frequency Fc is finished in step N39, a first search for the optimal cutoff frequency Fc, shown in FIG. 9B, is finished in step N40.
Turning back to step N5 shown in FIG. 8, a second search for an optimal cutoff frequency and a boost amount is performed only on a CD. FIG. 10 shows a flowchart illustrating the second search for an optimal cutoff frequency and a boost amount BA that is shown in FIG. 9C.
As shown in FIG. 10, it is decided in step N51 which of a DVD and a CD an optical disc for use in equalizer adjustment is. If the optical disc is a CD, initialization corresponding to a CD is performed in step N52. That is, a range, in which a cutoff frequency is searched for, is set in such a way as to have a certain width from a point, at which the searched cutoff frequency Fc corresponding to the optimal jitter is searched for, so as to search for a cutoff frequency Fc. Also, a range, in which a boost amount is searched for, is set so as to search for a boost amount. Thereafter, a search start point is set in the range in which the cutoff frequency Fc is searched for. Also, a search start point is set in the range in which the boost amount is searched for.
Subsequently, the cutoff frequency Fc is set at the search start point (as an initial value), which has been set as described above, in step N53. Then, jitter is measured in step N54. It is decided in step N55 whether the measured jitter is less than the optimal jitter. If so, the measured jitter is stored as the optimal jitter. Thus, an optimal cutoff frequency and an optimal boost amount at that time are obtained in step N56.
Upon completion of processing in this step N56, or if the measure jitter is or more than the optimal jitter, a search point for searching for the next cutoff frequency Fc is set in step N57. Similarly, jitter is measured in step N54. If this measured jitter is less than the optimal jitter in step N55, the measured jitter is stored as the optimal jitter in step N56.
Such a process is repeated until measurement of jitter corresponding to the last cutoff frequency Fc is performed. Thus, plural jitter values are stored. If processing corresponding to the last cutoff frequency Fc is finished in step N58, a next boost amount is determined instep N59. Upon completion of processing corresponding to the last boost amount in step N60, this second search for the optimal cutoff frequency Fc is finished in step N61. The apparatus returns to step N5 shown in FIG. 8. Then, the setting of the circuit for measuring jitter is canceled in step N6. Thus, this equalizer adjustment is finished in step N7.
JP-A-2002-343023, JP-A-2002-8243, JP-A-9-44997 and JP-A-2001-23167 disclose such techniques.